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Author Topic: 3dB downstream target SNRM  (Read 8520 times)

kitz

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Re: 3dB downstream target SNRM
« Reply #30 on: February 02, 2016, 05:44:21 PM »

Guys is there physical Retx in ADSL2 itself?

Its possible to do it on adsl2+,  but I think just on the downstream (dont quote me on that Im going from memory re upstream).   
So yes it is available, but BTw havent implemented it.
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Weaver

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Re: 3dB downstream target SNRM
« Reply #31 on: February 02, 2016, 05:52:31 PM »

Wow. Seems a shame, an opportunity missed. I can imagine it complicating the software somewhat because of the requirements of managing the buffering. It might be faster to disable the system in a world with no errors.
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kitz

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Re: 3dB downstream target SNRM
« Reply #32 on: February 02, 2016, 05:57:19 PM »

I believe it is perfectly possible to maintain a connection with a BER that is worse than the 0dB BER .  But I suspect the modem might be supposed to retrain at 0dB, on the assumption that such a connection was undesirable.

Please note careful, politician-style, choice of wording in above.. 'I suspect', 'I believe', etc.  Deduce that I'm not 100% sure of my facts... and not volunteering to download ITU specs and spend the rest of the week researching to confirm.    ;)


I think that it is more to do with the number of errors recorded rather than the actual value of the SNRm.   
The modem should go into an alarm state when 'x' no of errors per second is recorded.  If you look at LOS you can see the type of errors and alarm states which cause the modem to resync.

So yes you are correct in that it is possible to go into negative SNRm  - as some of the old netgears used to.   The E/Sec rate would get pretty high though and the connection would get really laggy.   This was back in the days before DLM monitored Errored seconds,  today's DLM would kick in pretty quick, and bump up things like interleaving - not just the target SNRm
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kitz

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Re: 3dB downstream target SNRM
« Reply #33 on: February 02, 2016, 06:05:46 PM »

Wow. Seems a shame, an opportunity missed. I can imagine it complicating the software somewhat because of the requirements of managing the buffering. It might be faster to disable the system in a world with no errors.

It may be something to do with the MSANs.   Bear in mind that some of BTs MSANs (such as the Marconi's) have now been in a long time and may not support a technology that is much newer than the equipment .   BT have lots of different makes of equipment for the MSANs so it could cause complications trying to update it on all.    The MSANs hold a lot more users than what the cab DSLAMs do, so it could also require a lot of buffering for those MSANs.

I think Sky have implemented retx on their ISAMs.  Certain SPs stick to certain equipment.  Funny enough I was in the exchange today and saw some Stingers.   I immediately knew that those would belong to Easynet... and they were.    Same as TT tend to use Huawei. 
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kitz

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Re: 3dB downstream target SNRM
« Reply #34 on: February 02, 2016, 06:19:05 PM »

@d2d4j - thanks for that John ;D You can tell how long your connection has been up, which is a great advantage over my situation. I have no idea, apart from hoping I can spot resync or reconnection events.

And yes please, pic would be good.

Here's something from me.  I think its from about 7yrs ago.   My line would sit at 3dB for months on end without any errors and sync at 24/2.2Mbps (Annex_M)

As an experiment I ran it at 2dB for a while and it would also perform quite happily without errors.  I then pushed it further to 1.5dB.   Here you can see I ran it on 1.5dB of SNRm for about 6 weeks.

This was a BE line so no automatic DLM, and the tweaks were done at a router level.  After that I couldn't be bothered to tweak and left it at the default 3dB.   I'll also attach my errored seconds for the same period.  These are cumulative and as you will see, after I cleared the totals in March, the amount of ErrSecs was practically zilch..  therefore I could probably got away with even lower than 1.5dB
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